Makefile should build for the _current_ target by default

[JssDWt]

When using the makefile, most commands should use the current target by default. If a developer wants to compile the python bindings, he should call make python for example, and it should work for the target he is on currently.

In order to get the current target, we could use TARGET=$(rustc -vV | sed -n 's|host: ||p'). (not sure whether that works on all platforms). There should be logic for copying the correct binaries to the correct locations. It could be .dylib, .so, .dll for example.

So

• use current target by default
• Make sure the commands work on all platforms (with the current target)
• Have specialist commands to build for specific platforms and combinations of platforms